1. Technical Field
The present invention relates generally to numerically controlled machine tools, and more particularly to the interruption of machining along a programmed path in response to the sensing of tool degradation or improper machining conditions. Specifically, the present invention relates to automatic tool retraction from an arbitrary point along a machining path when an interrupt occurs, and automatic return of the tool or a replacement tool thereafter to continue machining, in such a fashion that the retraction and return of the tool does not cause collision or interference with the workpiece being machined.
2. Background Art
It is known to automatically interrupt the normal execution of the part program of a numerically controlled machine tool in response to the detection of abnormal machining conditions. Various kinds of machining conditions have been monitored, and different actions have been taken in response to the abnormal conditions.
Many schemes have been devised for sensing different tool conditions based on the force applied to the cutting tool. The force can be measured directly, but it is usually more convenient to monitor the electrical power consumed by spindle or feed motors.
Fromson et al., U.S. Pat. No. 4,442,494 issued Apr. 10, 1984, discloses a tool wear and tool failure monitoring system that senses the second time derivative of the net power applied to the cutting tool in order to detect approaching tool failure, and a statistical representation of such occurrences with a series of tools is used to determine the percentage wear of a tool. In claim 4, column 10, the operation of the machine tool is said to be shut off when the second derivative of the net power increases beyond a critical level.
Olig & Ladwig, U.S. Pat. No. 4,509,126 issued Apr. 2, 1985, (also published Dec. 22, 1983 as PCT application No. PCT/US83/00847 and international publication No. WO83/04445), discloses an adaptive control system that senses electrical power supplied to a spindle drive motor for a lathe, calculates net machining power at the cutter tip and cutting efficiency (i.e., net power required for removal of a unit amount of workpiece material), and monitors cutting efficiency to perform elementary tool wear, tool breakage, and tool protection functions. Adaptive control of machining rate in response to the net machining power is inhibited for a programmed beginning distance A.sub.p and ending distance B.sub.p with respect to a cut through the workpiece. The beginning and ending distances A.sub.p, B.sub.p are said to be block constants or modal parameters. A "broken tool" feed hold is executed in response to the relative cutting efficiency dropping below a predetermined fraction of the initial cutting efficiency for the cut. A "tool protect" feed hold is executed in response to the relative cutting efficiency exceeding the initial cutting efficiency by a predetermined fraction. A "tool wear" feed hold is executed in response to a running average of the cutting efficiency exceeding the cutting efficiency of a sharp tool by a predetermined fraction.
A tool failure detection system advertised by Kabushiki Kaisha Komatsu Seisakusho (i.e., Komatsu Ltd.) is said to monitor the motor loads for the main spindle or feed axis, and judge whether each motor's load is normal. It is said that if a motor's load is judged abnormal, a safety cycle program functions to retrieve the tool being used to its tool changing position. The user is to press the start button after the tool with the abnormality is replaced. It is said that the new tool automatically returns to its original operating position, with cutting operations resumed.
The retraction of the tool from the workpiece and its subsequent return is itself a difficult operation for complex workpieces. For simple lathe machining tasks such as turning or facing, the tool can be retracted from the workpiece without collision or interference by incrementally moving the tool along a predetermined direction, and thereafter the tool can be moved directly to a tool change position. During retraction, the path traced by the tool can be stored in memory so that after a tool change the new tool can be returned to the workpiece and machining can be resumed where it was interrupted. In general, however, the path of the tool between the workpiece and tool change position must be chosen in response to the location of the tool when machining was interrupted. Otherwise, a collision or interference may occur between the tool and workpiece.
Ulrichsen and Brataas, U.S. Pat. No. 3,496,805 issued Feb. 24, 1970 discloses a method for automatic return movement of a toolholder along an arbitrarily chosen path, and new forward movement of the toolholder. The tool is moved back from an arbitrary point on the machining path to the tool change position by moving the tool along or parallel with the machining path in response to an auxiliary data set. From the tool change position the new tool is driven forward by the auxiliary data set to said arbitrary point, from which normal machining may resume.
Beadle et al., U.S. Pat. No. 4,055,787 issued Oct. 25, 1977 discloses a system wherein the tool may be promptly brought to a stop on the programmed machining path in response to any randomly timed feedhold signal, and thereafter moved under manual control away from the programmed path. The manually determined path is recorded, and in response to a restart signal the tool can be automatically returned to the interrupted point on the programmed path. The return path is determined by reverse retrace of the recorded path. Resumption of tool feed along the programmed machining path is inhibited until the tool is, in one way or another, returned to the interrupted point on the programmed path.
Wakai et al., U.S. Pat. No. 4,442,493 issued Apr. 10, 1984 discloses a system wherein a tool is automatically retracted from a workpiece, along a path determined in response to the current mode of the machining operation. Based upon information found in the program controlling the normal machining operations, the mode of the machining operation is determined to be either external diameter machining, edge face machining, or internal diameter machining. This information, combined with the position of the interrupted point along the machining path and the machining mode, and with a programmed parameter relating to the size of the workpiece, are said to be mathematically processed to determine a retract path to a first position of clearance from the workpiece and thence, avoiding contact with the workpiece, to a tool change position. It is further said that the cutting tool is returned to the normally programmed machining path at a point preceding, in sequence, the interrupted point in order to provide an overlapping of machining in that region. Tool retreat and return modes are stored on predetermined blocks of an NC tape. Transfer codes are also stored on the NC tape to select these modes. The transfer codes include, for example, M81, M82, and M83 corresponding to the external diameter machining mode, the edge facing machining mode, and the internal diameter machining mode. It is said that each mode includes variables representing retreat start position (i.e., the coordinates of the interrupted point), the retreat position, etc. Data for the retreat position is, for example, programmed into the NC tape or manually entered by an operator. An abnormal machining condition is sensed by comparison of feed motor current to a selected threshold current. A plurality of threshold currents are provided for respective portions of the workpiece and respective tools. The appropriate threshold is selected in response to "T codes" in the part program.
Kennametal, Inc. of Latrobe, Pennsylvania 15650 has developed a tool condition sensor for generating interrupt and tool worn signals, and has considered appropriate responses of a numerically controlled machine tool to the interrupt and tool worn signals. In accordance with the preferred responses, a worn tool is retracted and replaced at the end of the cut. Retraction of the tool includes an incremental move away from the workpiece followed by one or more secondary moves to the tool change position. The moves are programmable. For a broken tool, the preferred response is an immediate retract followed by a change of both the tool and the workpiece. These preferred responses were implemented on a Dynapath control using the macro capability of the control. During a demonstration of this system at the September 1984 International Machine Tool Show (IMTS) in Chicago, the tool changes were followed by a tool offsetting cycle using a vision system. General aspects of such a system were disclosed in Powell et al., "Sensing and Automation For Turning Tools," Technical Paper No. MS84-909, Society of Manufacturing Engineers, Detroit, Mich. (Apr. 5, 1984).
The Kennametal tool condition sensor is further described in a report titled "Electrical Interface Specification," Revision 2 (Nov. 30, 1984). The tool condition sensor monitors feed force, radial force and tangential force with respect to the tool, and transmits a tool worn signal and priority interrupt signals to the machine tool. The priority interrupts include a tool breakage interrupt, a transducer overload interrupt, and a fault interrupt. The tool breakage interrupt indicates that either tool breakage or tool chipping has occurred. The transducer overload interrupt indicates that abnormal machining conditions have been encountered. The fault interrupt indicates that a malfunction has occurred.
The suggested programming or configuration of the machine tool control for use with the Kennametal tool condition sensor is described in a report titled "Kennametal Tool Condition Sensor: Functional Specification," Revision 2 (August 1984). In particular, the machine tool control must initiate and execute a suitable recovery sequence in response to the tool worn or priority interrupt signals. The suggested recovery sequence for a tool worn sequence is to finish the cut, safe retract, change the tool, and continue cutting. If the tool is not used again for machining the current workpiece, however, the tool change can be delayed until machining of the current workpiece is finished. In response to a tool broken interrupt, an immediate "motion hold" is required, which should be followed by a safe retreat to the tool change position. To avoid any problems which may occur due to tool debris embedded in the unfinished workpiece, the recommended strategy is to change the workpiece and start machining from the start of the new workpiece. Also, in the special case of a threading operation, the tool must be retracted far enough to clear the threads before execution of the motion hold; otherwise, the existing threads will be ruined and further damage to the tool may occur. In a similar fashion, the recommended strategy in response to an overload or a fault interrupt includes an immediate motion hold and a safe retract, but thereafter a request for operator assistance is flagged. To implement these recovery strategies, the machine tool control must include a "logical jump on external trigger" feature and a safe retract feature. The logical jump on external trigger feature requires the capability of initiating a logical jump in the part program in response to an external trigger and keeping track of where the jump occurred so that execution may return and continue from the interrupted point. The safe retract feature requires the capability of safely moving the cutting tool from any arbitrary point in the part program to the tool change or turret index position.
The "Kennametal Tool Condition Sensor: Functional Interface Specification" considers alternative safe retract strategies. It is said that the absolute safe method is to run the part program in reverse to the previous turret index position. It is said that this may be difficult if not impossible to implement in existing controls but should be included as an integral feature in future system designs. It is further said that an alternative safe retract strategy, described in an Appendix A, should be simpler to implement although more cumbersome to use; moreover, it is said that it involves an increased part program burden and is not immune to programmer error. It is recognized that most modern machine tool controls provide some form of customer programmable macro capability including high level language functions. In accordance with the alternative safe retract strategy, it is suggested that this capability be exploited to write a general safe retract function which can be configured to particular situations. The macro call statement for the function will be placed in the part program block to which it applies and will contain the parameters necessary to configure the macro for the conditions pertaining to that portion of the part program. The general safe retract function will consist of a series of moves to get the tool back to the turret index position. The moves include (1) an incremental jog away from the workpiece; (2) optional X and Y moves to clear obstructions; (3) for outside diameter tools, an X followed by Z retract to the turret index position; and (4) for inside diameter tools, an X retract to the spindle center line followed by Z and X retracts to the turret index position. It is said that a fairly simple general safe retract macro can be constructed which is configured by a minimal number of parameters included in the call statement. A specific example is the call statement: EQU MACRO#(IF SET, ID/OD, ORIENT., JOG, X, Z, X, Z)
The parameter "IF SET" denotes the trigger which executes the logical jump to the macro, the parameter ID/OD specifies whether the cutter is inner or outer diameter, the parameter ORIENT. is the tool orientation used as direction for the incremental jog, the parameter JOG is the incremental jog distance, and the parameters X and Z are optional axis move parameters used to avoid obstacles.
The Kennametal general safe retract function invoked by the macro call is an example of a predefined stored program or "canned cycle" which is called up by the user's part program. Canned cycles are commonly supplied by a machine tool manufacturer to assist the user in programming repetitive functions as repeated calls to the canned cycles. Canned cycles can be general purpose as well as example, discloses the use of a microprogram executed specific to a particular machining path. Hotch et al., U.S. Pat. No. 4,446,525 issued May 1, 1984, for by a processor which recognizes parameters in the macroprogram as belonging to a set of parameters having values defined in a parameter table. During execution of the macroprogram, the processor obtains the respective values from the parameter table. Therefore, the machine operator may effectively edit part program blocks just prior to running them by changing selected values stored in the parameter table.